Study of genetic diseases with neoplastic manifestations and detailed investigations of families at high risk of cancer may help detect environmental and genetic influences in carcinogenesis, especially when appropriate laboratory assays are used, and may lead to important opportunities for cancer control. Thirteen women from five families had prophylactic mastectomy or sought counseling because of their high risk for breast cancer, usually because of a family history; a counseling strategy was devised that provided personalized summary estimates of risk and presented the options for preventing cancer deaths by medical surveillance or prophylactic mastectomy. Neurofi-bromatosis, an autosomal dominant disorder with a large predisposition to cancer, was linked to the serum protein marker, GC, on chromosome 4, by classical genetic linkage analysis in five families with a lod score of 2.2; because a sixth family gave a negative lod score, genetic heterogeneity may be present in this common but understudied preneoplastic disorder. A similar condition, the nevoid basal cell carcinoma syndrome, was found to have abnormal in vitro sensitivity to gamma-radiation, paralleling known in vivo radiosensitivity. Cells from patients with the hereditary dysplastic nevus syndrome had ultraviolet (but not gamma) radiosensitivity. Five case reports illustrated important lessons in cancer diagnosis and etiology, and suggested leads for definitive studies. For example, fulminant fatal eosinophilia in a man with lung cancer, several benign neoplasms, and a positive family history of cancer, was diagnosed as a rare eosinophilic leukemia because a new chromosomal abnormality, 15q-, was found. All cytogenetic abnormalities associated with human cancer were summarized in a figure that emphasized the genes assigned to specific chromosomes, including the newly described human oncogenes. Other literature reviews, guest lectures, and committee activities were done to stimulate similar research in the metropolitan Washington area and worldwide.